1. Field of the Invention
The present invention relates to a high frequency channel selecting circuit for a radio receiver and a method of manufacturing three tuning circuits for constituting the selecting circuit. This method is particularly suitable for a radio receiver, because inductance adjustment for tracking effected on a circuit board can be eliminated.
2. Description of the Prior Art
In a radio receiver of electronic tuning system, a high frequency channel selecting circuit is composed of an antenna tuning circuit, a high frequency tuning circuit and a local oscillation tuning circuit, and each tuning circuit is formed usually by use of a coil, two variable capacitance diodes, and a capacitor.
Therefore, when the channel selecting circuit as described above is formed on a circuit board of a radio receiver by use of discrete circuit elements, the inductance of each coil for forming each tuning circuit is previously adjusted at a central position of the variable range, and further, after the discrete circuit elements have been all mounted on the circuit board, the tracking adjustment is effected again.
In this tracking adjustment, the inductance of an oscillation coil is adjusted in such a way that a lower limit oscillation frequency of the local oscillation tuning circuit can be obtained at the lower limit tuning voltage of the variable capacitance diode and further, after that, the inductances of the antenna tuning circuit and the high frequency tuning circuit are both adjusted in such a way that the audio output can be maximized at a frequency referred to as a tracking point on the low band side of the received frequencies.
Further, there exists such a case that trimmer capacitors of both the antenna tuning circuit and the high frequency tuning circuits are adjusted in such a way that the audio output can be maximized at a high band side tacking point.
When the channel selecting circuit is formed on the circuit board by use of discrete circuit elements as described above, the three coils of the three tuning circuits must be adjusted three times or more at different frequencies respectively, in addition to the coil adjustments effected before the coils are mounted on the circuit board.
Therefore, when the trimmer capacitors are additionally adjusted, since the trimmer capacitors must be adjusted reciprocatingly between the low band side tracking point and the high band side tracking point, the coil inductances must be adjusted on the circuit board as large as seven to nine times.
In order to reduce the number of times of adjustments of the circuit elements on the circuit board, the antenna tuning circuit, the high frequency tuning circuit and the local oscillation tuning circuit are sometimes all assembled as a single block, and the tracking adjustment is effected under the same conditions as being mounted on the circuit board of the radio receiver, in order to eliminate the adjustment on the circuit board.
However, when the entire channel selecting circuit is assembled as a block, since the number of circuit elements to be assembled in the block increases and thereby since the circuit board on which the number of the circuit elements to be connected in the block increases, the cost of the selecting circuit is inevitably costly.
In addition, since the antenna tuning circuit, the high frequency tuning circuit and the local oscillation tuning circuit are all arranged close to each other on the circuit board, when the radio frequency becomes high as with the case of an FM radio receiver, the influence of radiation from the local oscillation tuning circuit becomes serious. Therefore, in order to prevent the mutual interference between the tuning circuits within the block, the circuit must be designed carefully, under due consideration of various restrictions in arrangement of the circuit elements and the wiring pattern.